Helmet



FIG. 1 is a perspective view of the design for a helmet;

FIG. 2 is a left view thereof;

FIG. 3 is a right view thereof;

FIG. 4 is a front view thereof;

FIG. 5 is a rear view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The dashed lines are for the purposes of illustrating portions of the helmet and form no part of the claimed design. 

CLAIM The ornamental design for a helmet, as shown and described. 